The invention relates generally to the art of coatings and more particularly to an improved thermal barrier coating for use on heat and oxidation sensitive metallic alloy systems.
Thermal barrier coatings are applied to hot sections of gas turbines or jet engines, such as combustor cans, nozzle guide vanes, and turbine blades. The function of such coatings is to increase engine efficiency by elevating the operational temperature or reducing the need for cooling air. The use of thermal barrier coatings in large turbines for land-based power generation is critically necessary for an acceptable operating lifetime. Because the superalloys of such turbines begin to melt at 1260.degree. C. to 1290.degree. C., it is necessary to use thermal barrier coatings and complex cooling mechanisms for today's turbine systems. The thermal barrier coating enables extension of component life by lowering the metal temperature. The engine reliability is increased by reducing the metal temperatures by 50 to 220.degree. C., which also increases the engine efficiency by reducing the cooling air requirements, reduces fabrication costs by eliminating elaborate cooling schemes and, most of all, provides significant performance improvement and thus large cost savings by increasing the turbine inlet temperatures.
The development of high-performance thermal barrier coatings for higher temperature use is driven by the demand for higher fuel efficiency. There is a need to produce thermal barrier coatings with lower thermal conductivities by generating greater porosity without significantly affecting the thermo/chemical/mechanical strengths of the coating, or by replacing yttria with other ceramic materials such as ceria. With the maximum gas inlet temperature at present of approximately 1420.degree. C., the use of today's ceramic thermal barrier coating system reduces the metal surface temperature of internally cooled airfoils by as much as 170.degree. C. This enables an increase in the maximum combustion temperature necessary for saving fuel by more than 12% without increasing the surface temperature of the metal substrate. Future developments aim at utilizing inlet temperatures of 1760.degree. C. which require improved, enhanced thermal barrier coatings. Control of the spallation behavior of the bond coat oxide by providing constraint effects due to the presence of a ceramic top coat that significantly influences the oxide growth kinetics and morphology, and by increasing the effectiveness of the oxygen barrier layers to reduce the oxide growth rate, is critically important to the performance of the thermal barrier coating system.
There is thus a continuing and pressing need for improved thermal barrier coatings so as to advance the efficiency and life of turbine systems.